This invention relates to a motor vehicle powered by an internal combustion engine which is equipped with an EGR system (a system for recirculating portion of the exhaust gases to the induction manifold of the engine for the purpose of reducing NOx emissions from said engine.) and more particularly to an improved EGR system.
It is well known to recirculate a portion of the exhaust gases emitted from the engine to the induction manifold to produce an air/EGR-gas/fuel mixture which exhibits a reduced rate of combustion in the combustion chamber or chambers of the engine. This reduced rate of combustion reduces the peak combustion temperatures to accordingly maintain the combustion temperature at a level where NOx is virtually not formed. However to achieve effectively the desired reduction of the NOx generation during combustion it is necessary to recirculate rather large amounts of exhaust gas. Combustion chambers have been developed which permit very high rates of EGR to be employed without loss of power output of the engine. However during certain modes of engine operation such as high speed and low load NOx is inherently low and accordingly a reduced rate of EGR can be used without any increase in NOx emissions. However an EGR system for controlling the rate of EGR has not been developed yet which can adequately proportion the amount of exhaust gases introduced into the engine during all modes of engine operation when a very high rate of EGR is employed. Systems which have attempted to achieve the above mentioned degree of control have suffered from various drawbacks such as over sensitivity to variations in exhaust gas pressure in the exhaust manifold and associated exhaust conduit. This so-called over sensitivity has not presented any large problems in EGR systems which recirculate exhaust gases in the order of a few percent. A variation in the amount of EGR gas supplied under the afore mentioned condition produces only a slight change in engine performance. However a small change when using upwards of a 50% EGR rate results in a large change in the amount or volume of exhaust gases in fact introduced into the combustion chamber or chambers. Erratic engine performance immediately results from this rather large change in the volume of EGR gas supplied thereinto. Thus despite the development of combustion chambers having the capacity to combust mixtures containing large quantities of EGR gas the engine as a whole has been unable to produce the desired performance.
There are several possible sources of the afore mentioned pressure variation, these include; accidents which bend or dent the exhaust pipe and or manifold to a degree where an increased back pressure results causing a higher than intended pressure to be present in the EGR passage; carbon deposites in critical places causing a reduction in the effective cross sectional areas of, for example part of the EGR passage; and variations in the dimensions of the exhaust manifold, EGR passage etc. during mass production of same. The latter of course always exists and there is inevitably a car to car variation in the pressure existing in the EGR passage immediately downstream of the valve controlling the flow of the gases through said passage.
Thus there still remains a need for an EGR control system which operates to adequately proportion the amount of exhaust gases recirculated during all modes of engine operation, which is not effected by the above described pressure variations and therefore controls the supply of large amounts of EGR gas to the engine in a manner which ensures optimal output of the engine with greatly reduced NOx emissions.